The invention relates to a holder for a cutting tool. In particular the invention relates to holder for a cutting tool where the holder is adapted to firmly hold the cutting tool to transfer a rotational movement from a driving source in boring or milling machine to a cutting tool secured in the holder. The cutting tool may preferably be designed for chip forming metal cutting.
Precision cutting requires that the position at which a cutting tool is located when held by a holder for the cutting tool is highly reproducible. For this purpose various types of holder for cutting tools are known. Precision hole making is defined by characteristics—among others—such as straightness, roundness and position accuracy, and in addition—for some applications—the properties of the produced surfaces. In particular for high integrity components such as gas turbine rotors and pressurized vessels a high demand is imposed on the tools in order to achieve the desired property on the machined surfaces. The type of tool holder used has an impact of the quality of the worked surfaces.
A common type of tool holder is designed as body having a channel for receiving a stem portion of a cutting tool. The stem portion will be locked by screws that penetrate into the channel in order to engage with the stem portion of the cutting tool. With this type of locking engagement, load is concentrated to one or two locations where the screw or screws engage with the stem. It has shown that this type of engagement does not suppress oscillations for certain frequencies, which frequencies depend on the location of the connection between the screw and the shaft. Another type of locking device is presented in U.S. Pat. No. 6,568,055, where an axially wedge shaped sleeve is pushed by a hydraulic actuator to tighten a boring arbour.
It is desirable to provide a tool holder that facilitates preparation of high quality surfaces and which creates conditions for a cost-efficient production and/or use of the tool holder.
A tool holder according to an aspect of the invention comprises a body having a first channel for receiving a stem of the cutting tool. The body furthermore includes a second channel intersecting with the first channel. A plunger is inserted in the second channel. The plunger has a third channel with a cross section which at least partly overlaps with the cross section of the first channel when the plunger is inserted into the body. The stem of the cutting tool thereby extends into the third channel when the stem is inserted in the first channel. The holder further comprises a mechanism for locking the stem against movement relative to the body by means of the plunger. An aspect of the invention is based on the observation that the quality of the machined surfaces is dependent on the wear of the cutting insert. By virtue of the fact that the holder comprises a plunger with said third channel, which plunger is configured for acting on the stem, an accurate holding of the cutting tool can be achieved. Further, by arranging the plunger moveable in the second channel which intersects with the first channel into which a cutting tool is inserted, lateral compensation of the position of the cutting tool is facilitated. Hence, production of high quality surfaces is facilitated.
According to an embodiment, the plunger comprises said locking mechanism. In this way, the need for including functional features for locking the cutting tool in the body is eliminated. It is therefore easier to make the body sufficiently sturdy to achieve a desired stiffness of the tool holder. Preferably the body may be formed as a monolith only provided with ducts for cooling liquid and means for locking and positioning of the plunger relatively to the body. Hence the tool holder enables production of high quality surfaces by facilitating lateral positioning of the cutting tool, while allowing a compact stiff design of the body. The plunger may thus be moveably arranged in the second channel so as to allow lateral dislocation of the plunger and hence of the cutting tool. Accurate lateral positioning of the cutting insert to compensate for the successive wear of the cutting insert may therefore be achieved in a straightforward manner while maintaining a firm grip of the cutting tool.
According to a further embodiment, said locking mechanism comprises a membrane delimiting a pressure chamber. The holder may comprise a mechanism for pressurizing the pressure chamber in order to lock the stem against movement relative to the body. In this embodiment a holder is provided, which enables suppression of oscillation over a wide range of frequencies.
According to a further embodiment, the membrane forms a part of the third channel and is configured to act directly on the stem. The pressure mechanism is configured to press the membrane directly against the stem. Thus, the membrane is integrated into the plunger, which creates further conditions for a firm locking of the cutting tool.
The contact between the stem and the membrane forming part of the third channel in the plunger ensures a distribution of the load on the stem enabling suppressions of oscillation of the cutting tool.
Optionally, the part of the third channel which is formed by the membrane is cylindrical and especially circular-cylindrical. The use of a cylindrical membrane ensures that the membrane grips the stem with a constant radial load. A hydraulic oil pipe is connected to the pressure chamber for enabling a movement of the membrane thereby enabling locking or release of the stem of the cutting tool. The use of hydraulic locking of the stem has a beneficial effect on the suppression of oscillations of the cutting tool.
Optionally a hydraulic oil pipe connects the pressure chamber with an outwardly facing end wall of the plunger, at which the mechanism for pressurizing the pressure chamber is located.
Optionally the first channel may extend along a first length axis and the second channel may extend along a second length axis. Each of the first and second channels are preferably straight. Optionally the first and second axes are essentially perpendicular to each other. By arranging the first and second axes perpendicular to each other, the risk of that the stem is axially dislocated during the locking process is reduced since a small movement of the plunger in the second channel in this case will not result in a translation of the plunger relatively to the length axis of the first channel. Hence movement of the plunger will not result in axial dislocation of the cutting tool.
Optionally the third channel extends through said plunger so as to enable a stem of a cutting tool to extend through said third channel when the stem is inserted in the first channel. The third channel is preferably straight. By allowing the stem to pass through the third channel a good contact between the stem and the membrane is ascertained.
Optionally the holder includes a mechanism for movement of the plunger along a length axis of said second channel so as to position the stem in a lateral position relatively to the body. Lateral positioning is performed to enable accurate positioning as the cutting insert wears out during use.
Optionally the holder comprises means for angular and/or axial positioning of the cutting tool relative to the body. Preferably, the positioning means comprises an indentation provided at an opening of the first channel, which indentation is arranged to receive a notch provided on said stem of the cutting tool, thereby enabling axial and angular positioning of the cutting tool.
The invention furthermore relates, according to an aspect thereof, to a cutting tool for use in a holder, which cutting tool includes a stem. The cutting tool comprises means for angular and/or axial positioning of the cutting tool relative to a body of the holder. Particularly said positioning means comprises a notch provided on a stem of the cutting tool, thereby enabling axial and angular positioning of the cutting tool in the holder. The notch is to be inserted in a corresponding indentation made at the holder for enabling axial and angular positioning of the tool in the holder.
Optionally the stem portion of the cutting tool includes a cooling liquid flow entrance in a lower region of the stem, a cooling liquid feeding conduit extending from said liquid flow entrance to a head located on the top of said stem, and a cooling liquid ejector outlet nozzle connected to said cooling liquid feeding conduit at said top portion of said stem.
Optionally the cooling liquid ejector outlet nozzle has a smaller cross sectional area than said cooling liquid feeding conduit. By arranging the cooling liquid feeding conduit with a larger cross sectional area than the cooling liquid ejector outlet nozzle it is ensured that the pressure drop in the cutting tool can be reduced.
Optionally the cooling liquid ejector outlet nozzle has a length between 1 mm and 3 mm. By making the cooling liquid ejector outlet nozzle short an output spray from the nozzle will be focussed. Long narrow channels will tend to result in a distributed spray. By making the length of the narrow cooling liquid ejector outlet nozzle short, the spray will thus be focused.
Optionally the cooling liquid feeding conduit is eccentric in relation to a centre axis of said stem. Steep changes in flow direction result in cavitation losses. The eccentric location allows for less steep change in flow direction in the vicinity of the cooling liquid ejector outlet nozzle.
The cutting tool preferably comprises a head located on the top of the stem. Optionally, said head includes a first surface, which extends in a plane in parallel with a longitudinal axis of the cutting tool, wherein a cutting insert is located on the first surface, and a second surface which extends in a plane which is inclined with regard to a plane at right angles to the longitudinal axis and which intersects with a length axis of the cooling liquid ejector outlet nozzle. This arrangement allows that the cooling liquid ejector outlet nozzle, which may have an exit opening in the inclined surface which extends perpendicularly to the direction of the inclined surface, may focus a cooling liquid jet on an edge of a cutting insert. The location of the rim an exit opening in a plane vertical to the length axis of the cooling liquid ejector outlet nozzle reduces spray diffusion in comparison to when the plane is inclined in relation to the length axis of the cooling liquid ejector outlet nozzle.
Optionally the cutting tool includes a cutting insert which before use has a polygonal cross sectional shape, that sides of the polygon meet at rounded corners and that a straight portion is present in the rounded area. Preferably, the polygon has the shape of an equilateral triangle or a square. Optionally the cutting insert is secured to said head such that said straight portion is coaxial with a centre axis of a stem of said cutting tool. By allowing the straight portion to be coaxial with the centre axis a reduction of wear of the cutting insert and of unintended deformation of the worked goods can be obtained. It is believed that in the event no straight portion is present in the rounded area a local concentration of force is obtained at the contact between the goods and the cutting insert such that the contact force will initially be too high with excessive wear and risk for damage of the cutting insert as well as of the worked goods as a consequence.
The invention also relates, according to an aspect thereof, to a cutting insert which before use has the shape of an equilateral triangle or a square, which sides meet at rounded corners and that a straight portion is present in the rounded area. Optionally, the rounded corners have a curvature radius between 0.6 and 1.2 mm and that the straight area has a length between 0.07 and 0.15 mm.